While Pennsylvania Senator Arlen Specter's announcement that he was switching from the Republicans to the Democrats may have gratified the latter and upset the former, no one should have been surprised by it. Historically, party switching is common, indeed inevitable, at times of party realignment.

Spurred by the rise of a new large dynamic generation and the emergence of a new communication technology, realignments occur about every four decades in U.S. politics.

The most recent one began with the election of Barack Obama last November. These large political makeovers normally enthrone a new dominant national party. Beneath the surface, realignments are characterized by major shifts in the voting coalitions that support the two parties. Demographic groups and regions move to and fro between the parties. These demographic shifts, and the attendant ideological solidification of the two parties, leave some politicians, like Senator Specter, who no longer represent their party's mainstream thinking, as outliers and essentially marooned.

Faced with the prospect of almost certain defeat by a reduced and much more conservative base in next year's Pennsylvania GOP Senate primary, the politically moderate Specter made the only rational choice he could and left the Republican Party.

The last previous realignment, which began with election of Richard Nixon in 1968, saw the movement of numerous Democrats, primarily Southerners, to the Republican Party. Starting with Strom Thurmond and continuing through Richard Shelby, these conservative Democrats, along with many of their constituents, saw the GOP as a more comfortable political home. But the movement between the parties wasn't only Southern, nor was it only in one direction. Conservative Colorado Democratic Senator, Ben Nighthorse Campbell, became a Republican and liberal Republicans like New York's John Lindsay and Don Riegle of Michigan went the other way.

If history is any guide, Arlen Specter's move from the Republican to the Democratic column will not be the last during the next several years. Other moderate Republicans, most likely from New England, the Northeast, and the upper Midwest, will almost certainly join him. At some point, it is also probable that a smaller number of conservative Democrats, who feel uncomfortable or politically threatened by President Obama's activist agenda, will find the GOP more compatible with their beliefs and fortunes. Just as the baseball offseason is a time for trading players, a political realignment leads to changes in the rosters of the two parties. Let the switching begin.